Many large vehicles have air-operated parking brakes (sometimes also referred to as safety brakes). Examples of vehicles include tractor trailers, tour and school buses, fire trucks, ambulances, garbage trucks, heavy construction equipment and recreational vehicles. Unlike normal air brakes, which use air pressure to apply a braking force, the parking or safety brakes use air pressure to prevent the application of braking force. Most typically, a powerful spring is used to provide the braking force, and air pressure acting against the spring keeps the brake disengaged. Thus, these brakes are fail-safe, meaning that they will automatically deploy in the event of a loss of air pressure.
Air-operated parking brakes are typically manually employed by the manipulation of a button or a lever controlling one or more air valves. To disengage the parking brake, the air valve(s) is/are operated to open a path from an air source (typically one or more reservoirs charged from an air compressor) to the operating mechanism of the brake. Air pressure then disengages the brake against spring pressure. To engage the parking brake, the valve(s) is/are repositioned so that the: a) the path to the air source is closed; and b) the operating mechanism of the brake is vented.
Despite the presence of such brakes, and their fail-safe design, theft and accidental movement of large vehicles equipped with them still occurs on a disturbingly frequent basis. Major shipping companies can lose several loaded tractor trailers a month. School, transit and tour buses are stolen for joy rides, or—when the driver leaves momentarily at a rest stop—rolling out of control, resulting in serious injury or loss of life to passengers.
The accidental movements can often be attributed to operator error. Although the parking brake will automatically engage upon loss of air pressure, under normal operating conditions, the driver or other vehicle operator must manipulate the button, lever or other mechanism to vent the air pressure and allow the brakes to apply. Some ideas for automatic venting have been put forward, but often not incorporated because of the risk that brake application will occur inadvertently while the vehicle is in motion.
Theft can be particularly problematic with large vehicles, because unlike personal cars or trucks, large vehicles are frequently not locked and often left running while parked. In fact, operating requirements for many such vehicles dictate that they be left running for short stops in order to minimize engine wear, conserve fuel, power necessary equipment, etc. In some instances, vehicle access requirements will also dictate against locking the cab of the vehicle. Combined with the fact that several large vehicles do not even have ignition keys, it becomes apparent that they are ripe for the picking.
As a theft prevention mechanism, it has previously been contemplated to apply a lock externally to the operating mechanism for the parking brake, so as to lock the mechanism in the vented position. However, such devices can be unwieldy and unduly complicate authorized vehicle use. Also, their external location can facilitate tampering and removal.